The present invention relates generally to permanent magnet generators, and more particularly to methods and apparatus for de-energizing permanent magnet generators.
Electromagnetic generators convert mechanical energy into electricity by producing changing magnetic flux from mechanical movement. Changes in magnetic flux through windings of wire induce voltage in these windings according to Faraday's law, producing deliverable electric energy. Permanent magnet generators (PMGs) produce varying flux through generator windings either with permanent magnet rotors, or with permanent magnet stators. Generators with permanent magnet rotors drive rotating permanent magnets to produce magnetic fields that vary at fixed stator winding locations. Generators with permanent magnet stators use stationary magnets to produce magnetic fields through which rotor windings travel, for the same ultimate effect. For simplicity, PMGs will hereafter be assumed to use permanent magnet rotors, although those skilled in the art will understand that generators can analogously be constructed with stationary magnets and rotating windings.
In some situations it becomes necessary to rapidly de-energize a generator, either for operator safety or to prevent damage to generator components. Operating an energized generator while shorts are present between generator windings, for instance, can cause rapid resistive burnout of generator wiring. When such harmful faults are detected, it is often desirable for generators to quickly and automatically de-energize to avoid further damage. It may also be necessary to de-energize a generator to avoid dangerous high voltages. Generators which produce flux with field windings rather than permanent magnets can rapidly de-energize by cutting off currents to field windings. PMGs, although desirable for many reasons, are more difficult to rapidly de-energize. So long as permanent magnet rotors continue to rotate, they ordinarily induce currents in stator windings. Rotating magnets produce varying magnetic flux governed by the right-hand-rule. Whenever net flux passing through stator windings varies, this induces currents tending to oppose the change in flux. As a result, a PMG will ordinarily continue to produce voltage as long as the permanent magnet rotor turns.
A variety of techniques have been developed to de-energize PMGs. A PMG can be de-energized by mechanically halting rotor movement, but this method is slow, and usually necessitates decoupling the generator drive shaft from its mechanical power source. As a faster alternative, some systems include specialized stator windings attached to an external power supply. When rapid flux cancellation is needed, these systems force current through the specialized stator windings to produce a countervailing magnetic flux, canceling changes in flux generated by the rotating permanent magnets. Another system (U.S. Pat. No. 7,777,384) provides conductive shunts which route flux away from stator windings, thereby preventing currents from being induced on stator windings. During normal operation, these shunts are saturated, and flux passes through the stator windings as usual. In a fault condition, the shunts can be “opened” by halting saturation, to de-energize the stator windings. A third system (U.S. Pat. No. 7,443,070) similarly uses shunts to route flux away from stator windings, but disconnects these shunts mechanically during normal generator operation.
PMGs are preferably capable of rapidly de-energizing without sacrificing normal operational power. As described above, existing mechanisms for de-energizing PMGs add considerable bulk or complexity. Simpler, smaller mechanisms are highly desirable.